[RoyalSlider error] No posts found matching your criteria time to time

Hi,
I have set up the slider and it works correctly most of the time, but in every 2-4 days the slider disappears and this message appears on the live website (which is not really nice, to be honest). I wonder what the problem can be. The page does use WPML to translate pages, but nothing in the widget area, also the posts are coming from a specific "slider" category that defines the picture, title, and caption (as a custom field) separately.